MITOSIS – Cell cycle, Stages, Significance

MITOSIS

Mitosis is important for sexual reproduction indirectly. It allows the sexually reproducing organism to grow and develop from a single cell into a sexually mature individual. This allows organisms to continue to reproduce through the generations.

CELL CYCLE:

Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides.

  • G1 phase: Metabolic changes prepare the cell for division. At a certain point – the restriction point – the cell is committed to division and moves into the S phase.
  • S phase: DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.
  • G2 phase: Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
  • M phaseA nuclear division (mitosis) followed by a cell division (cytokinesis).

The period between mitotic divisions – that is, G1, S and G2 – is known as interphase.

Mitosis:

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.

The replicated chromosomes are attached to a ‘mitotic apparatus’ that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.

In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase

STAGES:

Prophase:

Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.

Prometaphase:

The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell – at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.

Metaphase:

The chromosomes align themselves along the metaphase plate of the spindle apparatus.

Anaphase:

The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart – or ‘disjoin’ – and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)

Telophase:

The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.

Cytokinesis:

The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase – the interval between mitotic divisions.

SIGNIFICANCE:

1. It is an equational division through which identical daughter cells are produced having the same amount and type of genetic constitution as that of the parent cell.

2. It is responsible for growth and development of multi-cellular organisms from a single-celled zygote.

3. The number of chromosomes remains the same in all the cells produced by this division. Thus, the daughter cells retain the same characters as those of the parent cell.

4. It helps the cell in maintaining proper size.

5. Mitosis helps in restoring wear and tear in body tissues, replacement of damaged or lost part, healing of wounds and regeneration of detached parts (as in tail of a lizards).

6. It is a method of multiplication in unicellular organisms.

7. If mitosis remains unchecked, it may result in uncontrolled growth of cells leading to cancer or tumour.

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